When light is absorbed by an organic compound (organic molecule), the organic compound (organic molecule) gets to have energy (an excited state). Through this excited state, various photochemical reactions may be developed or light emission (luminescence) may be generated, and various applications have been tried. In particular, as an application field of a light-emitting compound, an electroluminescent device (a device that emits light by applying electric field) can be given.
In the case of using an organic compound as a light emitter, the emission mechanism of an electroluminescent device is a carrier injection type. In other words, by applying a voltage with an electroluminescent layer sandwiched between electrodes, an electron injected from a cathode and a hole injected from an anode are recombined in the electroluminescent layer to form a molecule in an excited state (hereinafter, referred to as an exited molecule), and energy is released to emit light while the excited molecule moves back toward the ground state.
In the foregoing electroluminescent device, the electroluminescent layer is generally formed of a thin film about below 1 μm. Further, since the foregoing electroluminescent device is a self-luminous device, where the electroluminescent layer itself emits light, a backlight that is used for a conventional liquid crystal display is unnecessary. Therefore, a great advantage of using such devices is that it is possible to manufacture a significantly thin and a lightweight display.
In the case of an electroluminescent layer with a thickness approximately from 100 to 200 nm, for example, the time from an injection of carriers to their recombination is about several ten nanoseconds considering the carrier mobility of the electroluminescent layer. Even when processes from the recombination of the carriers to light emission are included, only time on the microsecond time scale is required to reach light emission. Thus, the fairly high response speed is also one of the features.
In addition, since the foregoing electroluminescent device is a carrier injection type light-emitting device, driving by DC voltage is possible, and it is unlikely that a noise is generated. As for the driving voltage, an uniform ultra thin film with a thickness of approximately 100 nm is first used as the electroluminescent layer, a material for an electrode is also selected to reduce a carrier injection barrier against the electroluminescent layer, and a heterostructure (two-layer structure) is additionally introduced. Accordingly, a sufficient luminance of 100 cd/m2 can be obtained at 5.5V (refer to Non-Patent Document 1, for example).
(Non-Patent Document 1)
C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)
The electroluminescent device has been attracting attention as a next-generation flat panel display in terms of the features such as the thin thickness and the lightweight, the high speed response, and the low DC voltage drive. In addition, relatively favorable visibility can be obtained since the electroluminescent device is a self-luminous device that has a wide viewing angle, and the electroluminescent device is considered to be effective as the device for a display screen of a portable device.
By the way, light emission observed in the foregoing electroluminescent device is a luminous phenomenon in an excited molecule moving back toward a ground state. When excited, the molecule formed from an organic molecule can take two kinds of state: a singlet excited state (S*) and a triplet excited state (T*). In addition, the statistic generation ratio in an electroluminescent device is considered to be S*:T*=1:3 (see Non-Patent Document 2, for example).
(Non-Patent Document 2)
Tetsuo TSUTSUI, Text of the third lecture meeting, Bulletin of Organic Molecular/Bioelectronics Subconirnittee, Society of Applied Physics, p. 31-37 (1993)
However, in the case of a general organic compound, luminescence from the triplet exited state (phosphorescence) is not observed at room temperature while only luminescence from the singlet exited state (fluorescence) is observed generally. This is because T* (S0 transition (phosphorescence process)) becomes a forbidden transition and S* (S0 transition (fluorescence process)) becomes an allowed transition since an organic compound generally has a ground state of a singlet state (S0). In other words, only the singlet excited state contributes to luminescence generally.
Consequently, the internal quantum efficiency (the ration of photons generated to injected carriers) in an electroluminescent device is assumed to have a theoretical limit of 25% in accordance with S*:T*=1:3.
Further, generated light is not all coupled out to the outside, and it is not possible to take a portion of the light out due to inherent refractive indexes of a constituent material of an electroluminescent device and a material of a substrate. The ratio of light taken out to the outside to generated light is referred at as a light-extraction efficiency, it is said that the light-extraction efficiency is only about 20% in an electroluminescent device that has a glass substrate.
Consequently, it is said that the ratio of photons that can be taken out finally to the outside to the number of the injected carriers (hereinafter, “external quantum efficiency”) has a theoretical limit of 25%×20%=5% if injected carriers are all formed into excited molecules. In other words, if all of the carriers are recombined, it is conceivable that only 5% thereof can be taken out as light.
However, in these years, an electroluminescent device that is able to convert an energy released in moving back toward a ground state from a triplet excited state (T*) (hereinafter, referred to as “triplet excitation energy”) into light emission has been made in public one after the other, and a high luminous efficiency thereof has been attracting attention (refer to Non-Patent Document 3 and Non-Patent Document 4, for example).
(Non-Patent Document 3)
D. F. O'Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, “Improved energy transfer in slctrophosphorescent devices”, Applied Physics Letters, vol. 74, No. 3, 442-444 (1999)
(Non-Patent Document 4)
Tetsuo TSUTSUI, Moon-Jae YANG, Masayuki YAHIRO, Kenji NAKAMURA, Teruichi WATANABE, Taishi TSUJI, Yoshinori FUKUDA, Takeo WAKIMOTO and Satoshi MIYAGUCHI, “High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive center”, Japanese Journal of Applied Physics Vol. 38, pp. L1502-L1504 (1999)
A porphyrin complex that has platinum as a central metal and an organometallic complex that has iridium as a central metal are respectively used in Non-Patent Document 3 and in Non-Patent Document 4, and both of the complexes are of a phosphorescent material that has a third-series transition element introduced as a central metal, which includes one that well exceeds the foregoing theoretical limit 5% of the external quantum efficiency. In other words, an electroluminescent device using a phosphorescent material can achieve a higher external efficiency than conventional one. Then, as the external efficiency is higher, the luminance is improved.
Therefore, an electroluminescent device using a phosphorescent material is considered to occupy an important position in a future development as a means for accomplishing luminescence with a high luminance and a high luminance efficiency.
As described above, a phosphorescent material is useful for being applied to electroluminescent devices, which is expected. However, it is still the case that the number thereof is small. The iridium complex that is used in Non-Paten Document 4 is one of organometallic complexes referred to as an orthometalated complex. Since this complex has a phosphorescence lifetime of several hundreds nanoseconds and a high phosphorescence quantum yield, the decrease in efficiency associated with an increase in luminance is small as compared to the porphyrin complex. From that viewpoint, the foregoing organometallic complex using the heavy metal is one of guidelines for synthesizing a phosphorescent material.
By the way, an electroluminescent device has been actively developed for being applied to displays. Above all, the development of a device that can emit white light has been distinctly attracting attention. This is because full color display is possible when a color filter is attached to a display device, in addition to applications as mono-color display and lighting such as a backlight.
Since a light-emitting device with a filter passed through has an usability of light lowered, a device that can achieve high-luminance while light emission with lower power consumption is strongly required. In addition, considering the application as lighting, it is no mistake that a higher luminance is required. Therefore, it can be said that the use of an electroluminescent device using a phosphorescent material for realizing white light emission is the most effective means.
In the case of a conventional electroluminescent device using a phosphorescent material for white light emission, a method such as a method of mixing materials that respectively emit R (red), G (green), and B (blue) to form a thin film, a method of laminating layers that respectively emit R (red), G (green), and B (blue), or a method of combining complementary colors (a blue-green color and an orange color, for example) (refer to Non-Patent Document 5, for example) is considered. However, since a plurality of luminescent materials that have different emission wavelengths are needed in any case in order to obtain a spectrum of white light, there is a problem that a driving voltage is increased. In addition, since it is necessary to combine a hole blocking layer in order to obtain a high luminous efficiency from an electroluminescent device using a phosphorescent material, a quite complicated device structure is required.
(Non-Patent Document 5)
Brian W. D'Andrade, Jason Brooks, Vadim Adamovich, Mark E. Thompson, and Stephen R. Forrest, “White Light Emission Using Triplet Excimers in Electrophosphorescent Organic Light-Emitting Device”, Advanced Materials, 14, No. 15, 1032-1036 (2002)
As above, since a planar source for white light emission_is composed of a plurality of materials combined quite intricately, a manufacturing process is not only controlled with difficulty but also is made cumbersome and complicated.